Social Points are a new type of currency that became available after the new Neighbors update was installed. The are similar to the Lifestyle Points, but are purple in color instead.
You can get them by simply purchasing them from the Online Store. Get them by completing social goals at a neighbor's SimTown. You can also plant a social point flower (5 LP) on a garden patch, after you have the simoleon sprout.
Before the new Neighbors update, things that cost NBR did not deplete your NBR number since it was based on the number of Neighbours you had. However, buying things with the new Social Points system DOES deplete the number of Social Points you have, so you have to spend them wisely!